Angle measuring instruments are in industrial use on rotary and spindle axes, on electrical motors and especially on servomotors for controlling and regulating, as well as for positioning, especially in the machine tool industry and in manufacturing and handling mechanisms.
According to the state of the art, in order to measure the angle, one illuminates an optical dimensional standard, also known as a scale, preferably with parallel collimated light. The scale consists of lithographically produced chrome structures on a glass or plastic carrier. An angular movement of the scale relative to a sensor modulates the light impinging on the sensor. The angle information is present in binary encoded form on the scale, is detected by the sensor and is converted into electrical signals.
There are two kinds of angle encoding: incremental and absolute codes. Incremental codes are periodic line codes that are counted and thus document annular changes. However, they do not indicate the absolute angular position. Therefore, the scales often have a reference marking on a separate code track which, when moved across, determines a reference angle position, a so-called null position, and such scales are known as quasi-absolute. If one wishes to determine the absolute angular position, one incrementally counts back starting from the null position.
In contrast, so-called absolute angle measuring instruments always have a definite position, since the scale contains binary encoded position information. Common codes include, besides a binary code, which is encoded according to the law of formation of binary numbers, also a gray code or a random code. Codes formed according to the nonius or vernier technique are also used in industrial measuring systems. For this, line divisions with slightly different period lengths are arranged on at least two code tracks which, when sampled jointly, furnish an absolute position code.
A dimensional standard according to the Moiré principle consists of light and dark areas, formed by a photolithographically structured chromium layer deposited on a carrier substrate that is generally made of glass or plastic, which define an amplitude lattice that modulates the amplitude of the light.
For the scanning of the scale, a diaphragm is arranged in front of the electro-optical sensors, which consist of photosensitive semiconducting layers and which can be integrated on a chip if desired, or one can use semiconductor surfaces structured according to a dimensional standard.
The dimensional standard (10) is illuminated by an illuminating unit which has a light source (20), generally a light emitting diode (LED) emitting in the infrared spectrum, as well as collimator optics (30) for parallel bundling the emitted light. The collimator optics (30) usually is a glass lens, although it can also be made of plastic. The illumination unit is generally firmly joined to a stator.
In contrast to glass lenses, lenses of plastic materials can be more economically mass produced since a spherical or aspherical surface configuration of plastics is easier to attain than one of glass. Furthermore, plastics are easier to handle and grinding or polishing of the lens surface afterwards is unnecessary. Oftentimes the optical quality of injection molded plastic lenses is adequate for collimator optics of low luminous efficiency.
However, the production of plastic lenses and the fabrication of devices which contain plastic lenses present some problems. Plastic lenses are generally produced by injection molding. The plastic lens must have a shoulder so that it can be grasped and installed. Furthermore, mechanical stresses occur inside the plastic material during the fabrication of the lens because the spraying point merges directly in the lens surface, which afterwards can result in stress crack corrosion when subjected to temperature cycles, mechanical loading, or a solvent atmosphere. Therefore, a press-fitting or even a gluing of the plastic lenses when installing them in rotational angle measurement systems are not without problems.
In the case of the rotational angle measuring instruments described in German patent DE 197 50 474 C2 and U.S. Pat. No. 6,359,409 B1, the sensor is arranged concentrically over the shaft and the illumination unit is installed centrally in the shaft of the instrument and rotates with it. In such a case, the required energy has to be furnished by an appropriate mechanism. This can occur inductively, capacitively, or with slip contacts. Parts of the energy transmission mechanism (e.g. slip rings, capacitor plates or a coil with a coil former) are electrically connected to the illumination unit and also rotate with the shaft of the instrument. In the case of a capacitive or inductive energy coupling, additional electronic components such as rectifier diodes, capacitors, etc. are integrated in the illumination unit outside of the LED. Preferably, the electronic components as well as the light source are placed on a board. The light source is then preferably arranged as a Chip-on-Board (COB). For this, the non-encapsulated chip is directly bonded to the board and then encapsulated with a transparent substrate.